In the recent years, there has been an increasing demand in knit articles containing elastic yarn or rubber yarn, like so-called uncovered elastomeric yarns, these knit articles being surgical hoses, swimsuit fabrics, ladies stockings, socks and elastic body stockings, which have become very fashionable in the recent time. These knit goods are usually comprised of a certain percentage of elastic yarn and a certain percentage of non-elastic yarns. Usually, these goods have a size varying along the length of the goods. Often, these goods have an essentially tubular shape with varying diameter. One of the main problems in the knitting of these goods consisting of elastic fabrics is to achieve a predetermined form and size of the good being stable from product to product. Variations in the shape and size of the goods are caused by varying stitch densities in fabrics caused by numerous variable factors in the knitting process, for example caused by a varying yarn tension.
In the recent years, knitting machines for elastic yarns are usually equipped with yarn feeding devices for positively feeding predetermined quantities of elastomeric yarn to the working needles in the respective knitting station in the knitting machine. The term of positive feeding means that the yarn is not withdrawn from a yarn supply spool due to tensions in the yarn caused by the knitting process of the working needles, but means that the rate of withdrawal of yarn from a yarn supply spool is determined by the operation of the yarn feeding device, usually working in synchronism with the knitting machine.
Most of the prior art feeding devices for positively feeding elastomeric yarn to knitting stations of the knitting machine are mechanically coupled to a main motor drive of the knitting machine be means of a pin wheel and a belt drive mechanism for ensuring a synchronus operation of the respective feeders and the main drive motor of the knitting machine. A predetermined ratio of the yarn feeding speed with respect to the operational speed of the knitting machine can be adjusted by making use of a so-called quality wheel determining the belt speed for driving the feeders with respect to the rotational speed of the main shaft of the knitting machine by mechanically changing the diameter of the quality wheel. A variation in the feeding speed with respect to the operational speed of the knitting machine results in a variation in the stitch density and in turn in a diameter variation of the tubular good made by the knitting machine.
Feeding devices mechanically coupled to the knitting machine as described above have been manufactured and widely distributed by the applicant.
A similar prior art is examplified by GB-A No. 737561, GB-A No. 861880 and by GB-A No. 1380224.
GB-A No. 1380224 discloses a knitting machine having feeding devices mechanically coupled to the main drive of the knitting machine by means of a driven friction wheel allowing a changing of the gear ratio between the driving and the driven friction wheel for changing the knitting density. The gear ratio is determined by a mechanism coupled to a needle cam, the position of which determines the knitting density of the knit fabric.
Aktiebolaget Iro Wricehamn, Sweden distributes a yarn feeding device adapted to be mechanically coupled and driven by the main drive motor of the knitting machine by means of a pin wheel and a belt drive mechanism. This prior art yarn feeding device is called "IRO IEPF YARN FEEDER" this yarn feeding device is comprised of a vertical yarn spool carrier for carrying a spool of yarn adapted to be rotatable about an axis. The spool is pressed against a rotatable, vertical cylinder which is driven by the pin wheel engaging the drive belt.
It is known from DE-A No. 3002311 to make use of an electric motor drive for feeding elastic yarn from a still-standing supply spool to the knitting stations of a knitting machine. The prior art yarn feeding device is controlled by a microprocessor connected to a speed sensor for sensing the operational speed of the knitting machine. Although this knitting machine having a mechanically independent drive of the yarn feeder with respect to the main drive of the knitting machine is regarded as being advantageous since it is not restricted in the scope of positioning the feeding devices with respect to the remaining knitting machine, it has turned out to be unacceptable for high-speed implementations because of a lack in feeding-accuracy resulting in undesired changes in the mesh size or stitch density of the elastic fabric knitted by said machine.
The prior, non-prepublished European Patent application No. 161853 describes a knitting machine having electrically driven yarn feeding devices equipped with variablespeed electric motors driven by programmed controlled units connected to a speed sensor for detecting the operational speed of the knitting machine. A single electric drive motor drives a quality wheel which in turn engages a drive belt driving numerous feeding devices having the usual mechanical structure.